1. Field of the Invention
This invention relates to coatings of electroplated palladium-nickel alloys and in particular to a plating bath process for controlling the stability of the alloy composition in such coating over a wide variation of electroplating current densities.
2. Description of the Prior Art
Gold has historically been the plating material of choice for electrical contacts because of its resistance to corrosion, good solderability properties and low electrical contact resistance at low loads. Since gold platings are expensive, lower cost substitutes have been sought.
Alloys of palladium-nickel have shown much promise as a gold substitute for plating electrical contacts. One of the more successful such palladium-nickel coatings is described in U.S. Pat. No. 4,463,060 granted July 31, 1984 and assigned to the same assignee as the present invention, the disclosure of which is hereby incorporated herein by reference. The palladium-nickel electroplated surface coating described in this patent effectively protects the substrate from corrosion, is permanently solderable and exhibits reduced electrical contact resistance at low loads.
The coatings in the aforenoted U.S. Pat. No. 4,463,060 patent are prepared by electroplating in a bath of palladium (II) ammine chloride, nickel ammine sulfate, a small amount of brighteners, and a conductive salt. Electroplating is carried out at a current density ranging from about 5 to 25 amps/sq.dm., or 50 to 250 amps/sq.ft. (asf). At current densities in the upper portion of this range, above about 100 asf, the Pd-Ni composition of the plated coating can be fairly readily controlled. As current densities decrease below this level, controlling the alloy composition becomes increasingly difficult.
Controlling the Pd-Ni alloy composition during electroplating is extremely important. The properties of Pd-Ni alloy coatings which are important for electronic connector applications, such as solderability, ductility, hardness, thermal stability of contact resistance and environmental corrosion resistance, vary significantly with fluctuation in electroplated alloy composition. Accurate control of the level and constancy of plated Pd-Ni alloy composition is therefore necessary to assure the desired properties of connector products.
Other plated connector performance criteria, such as absence of porosity and wear life, are strong functions of the precious metal alloy thickness. All widely used, nondestructive, production techniques for measurement of precious metal alloy coating thickness, such as electron backscatter spectroscopy and x-ray fluorescence, must be standardized for a specific alloy composition. Therefore, in the production of Pd-Ni alloy plated connector products, the stability of the alloy composition is also necessary to achieve the desired control of alloy coating thickness and the properties that are thickness-dependent.
Of particular concern is the stability of the alloy composition as a function of current density. In the commercial plating of formed terminals, there can be variations in current density as high as a factor of four depending upon location on the connector. The magnitude of the current density variation is dependent upon the part geometry, the plating cell design, and other factors. A typical range of current densities for most formed terminals is 25-100 asf. Locations on a few terminals might be plated as low as 10 asf or as high as 150 asf.
The effects of current density variation upon alloy composition stability can be better explained with reference to FIG. 1 and Examples 1-3 below. For the purpose of this invention, the stability parameter for evaluation of Pd-Ni alloy plating process performance is defined as the difference between the Pd content in weight percent of an alloy deposited at 100 asf and that for an alloy deposited at 25 asf. This difference, which is illustrated for Curve A on FIG. 1, will be referred to and indicated by the symbol .DELTA. Wt% Pd.sub.(100-25). Plating baths formulated with typical commercially available palladium ammine chloride salts and organic brightener systems have a .DELTA. Wt% Pd.sub.(100-25) in the approximate range of 12 to 22 as shown in Examples 1, 2, and 3. In Example 1, identical plating runs with respect to bath chemistry and plating conditions were conducted with palladous ammine dichloride salts from six different commercial sources. The .DELTA. Wt% Pd.sub.(100-25) for these runs ranged from 13.0 to 18.7, indicating that they were all unstable with respect to the desired constancy of alloy composition.